The invention is directed to gold-silver alloys with good tarnish resistance for the dental art, particularly to castings of crowns, bridges and inlays. In connection therewith the metal additives are so chosen in their composition that danger of dark discolorations in the milieu of the mouth due to the addition of copper and/or silver is greatly reduced or entirely eliminated.
Gold alloys are preferred in the dental art for the production of crowns and bridges, which alloys for aesthetic reasons should be gold to light gold in color and whose hardness value after the casting for crowns should be at least 120 HV and for bridges at least 150 HV. This hardness value for a long time has been reached through addition of platinum metals and above all through addition of copper, which from the miscibility gap commencing from the system silver-copper makes possible a hardening mechanism in the crystal lattice by suitable heat treatment or by slow cooling after the casting. On the other hand copper containing alloys have the disadvantage that in the milieu of the mouth they lead to brown, black or bluish tarnish layers which mainly consist of copper sulfide and are strongly detrimental to the appearance of a prosthetic piece of work. This appearance occurs after a short carrying time in the mouth if there occurs a thin copper precipitate on the alloy surface because of local element formation which are discolored through the influence of sulfur containing compounds in the saliva and food. The copper comes from the copper containing solid solution crystal of the alloy, which is resistant itself. The copper diffuses out with oxide formation in the heating in air and collects on surface or in solidification cavities. It is true that this copper oxide goes into solution in the acidification, however, it can in using a non-noble metal forceps again deposit on the surface of the alloy. Besides the copper oxide on the inside of the cavities is frequently not dissolved out or only incompletely dissolved by acids, above all if cavities are first opened after the acidication of the cast piece through slipping of the cast piece. In those cases the copper oxide frequently first goes into solution in the course of weeks and months after incorporation of the piece in the mouth of the patients, during which local cells which form because of the differential aeration between inner and outer regions of the cavities lead to a further precipitation of copper ions as copper films in the vicinity of the cavity opening.
The danger of discoloration caused by the copper is eliminated if there is relinquished the inclusion of copper in the alloys and to produce the required hardness there is employed, in place of copper, additions of cobalt, iron and/or nickel (Kropp, German OS No. 2,136,232), indium tin and zinc (Wagner German AS No. 2,139,331) or increased additions of platinum and zinc (Kropp German AS No. 2,509,476 and related Kropp US.S Pat. No. 4,007,040). Alloys of the type mentioned have mouth and tarnish resistance but additions of cobalt iron and nickel exhibit the difficulty that the alloys are coated with oxide layers in the casting or annealing which are only dissolved with difficulty with the customary dilute acids (hydrochloric acid, dilute sulfuric acid or sulfamic acid) of the dental art. The increased addition of platinum leads to an undesired increase in cost of the alloys as well as to a relatively strong lightening of the gold color even at high gold contents. Both disadvantages above all are of importance if, to save expense, the gold content should not exceed a certain height. The complete elimination of the addition of copper in hard gold alloys which owe their hardness only to addition of platinum metals in combination with zinc, tin or indium proves disadvantageous in that the ductility of the copper free alloys is clearly lower than of the copper containing alloys, i.e., the danger from fractures by de- in formations is greater. Furthermore copper additions offer the advantage of a deeping of the gold color. For these reasons it is desirable in the dental art to use gold alloys which still contain a certain amount of copper, however, without the danger of a discoloration being exposed.
It is known that when pure copper is annealed in air it is coated with black copper oxide while copper-zinc alloys having a high zinc content (brass) with the same annealing treatment remain light, since in this case only white zinc oxide forms on the metal surface, while the formation of copper oxide is suppressed. The known gold dental alloys with relatively high amounts of copper and relatively low amounts of zinc upon annealing are coated with black copper oxide.
Furthermore, it is known that the susceptibility of silver and silver alloys to tarnish from sulfur compounds can be substantially reduced by additions of indium in combination with additions of tin and zinc (Harigaya, German OS No. 2,160,721) without the light silver color being impaired by a gray streak to a mentionable extent.
Therefore it was the problem of the present invention to provide gold-silver alloys with good tarnish resistant properties for the dental art, particularly for the casting of crowns, bridges and inlays which are relatively low in gold, nevertheless have a full gold color, are hard and nevertheless should be ductile.